Many tests have been devised to determine the interaction of various agents and tissues of humans and animals.
Tissue as used herein comprises any group or layer of cells which together perform one or more certain functions. Tissue includes, but is not limited to, epithelial tissue, connective tissue, cartilage, bone, blood, organs, glands and blood vessels.
The effects of chemicals found in the environment, both on man and animals, is of widespread concern. The effects of new drugs, both veterinarian and human, are routinely tested in accordance with federal regulations. Chemical companies, petroleum and paint companies pharmaceutical companies, and cosmetic companies use test systems to assay the reaction of skin to the substances they use and produce. In addition, biomedical laboratories in pharmaceutical companies, hospitals, and universities use test systems for the study of disease mechanisms and for the evaluation of treatment procedures.
At present, systems for determining the interaction of tissues and agents include (i) experimental animals (mainly rodents and rabbits), (ii) monolayer cultures of human cells, (iii) tissue slices or organs from cadavers, and (iv) mathematical models developed to simulate biological responses. Each of these test systems has its advantages and its shortcomings.
Experimental animals (excluding human subjects) have been widely used. Because the cells and tissues of these animals are different from those of humans the use of experimental animals to determine the effect of various agents on man is limited. Furthermore, experimental animals are expensive to maintain, and there are ethical considerations associated with the use of animals for such purposes.
Cultures of cells are highly reproducible, inexpensive, well standardized test systems, but they do not mimic the state of cells and tissues in the organism. As a consequence, the biosynthetic activities and physiologic functions expressed by cells grown in monolayer cultures are markedly different from those in the organism, thus yielding misleading test findings.
Tissue slices from cadavers can provide both the complexity as well as the normal biosynthetic output and cell properties needed for a test system capable of mimicking human responses; however, they are moribund. Some cells are alive, others are dying, and many are already dead. This limits their usefulness since, for example, in a toxicity assay, it may be difficult to distinguish between the effects of the test substance and the natural degenerative changes occurring in the cadaver.
Mathematical models are useful when responses are well understood and predictable and when the full range of variables is defined but they are not appropriate for testing new substances.
From the point of view of human health protection, the ultimate test organism is of course the human; however, human testing is subject to stringent limitations. Animals are widely used in testing because they can be dissected and probed invasively, and because they can be used for substances known to be toxic to humans; however, as mentioned previously their responses do not necessarily reflect human responses.
The skin, a very important tissue, is the principal barrier between the organism's internal milieu and the chemical and physical world without. It is thus subject to the ravages of the environment. It is exposed to agents, such as, chemicals and antigenic substances, in the workplace, in the home, and in the atmosphere generally. Medicaments are applied to the skin both for the treatment of systemic conditions by topical therapy, as well as for the management of wounds and numerous disorders that afflict the skin itself. The skin is treated cosmetically to improve its appearance and sometimes its health. Today there is broad concern with the necessity of establishing safe practices to protect the individual against the effects of intrusive and injurious substances that come into contact with the skin and to evaluate the effects of cosmetic and remedial emollients that are applied to it. Thus, it is not surprising that many tests have been devised to determine the interaction of various substances and human skin.
Skin testing on humans is limited primarily to tests of a "benign" character dealing with sensitization. For example, when human subjects are used to evaluate the effect of test substances on the skin, the skin responses monitored are usually erythema and edema. These are gross manifestations of complex processes that have well defined immunochemical, biochemical, and physiological counterparts at the cellular level. To analyze for such effects requires invasive procedures that are frequently inappropriate.
Although excised cadaver skin has been used for skin testing, it is not readily available and it rapidly becomes moribund. As it degenerates, the skin loses its capacity to respond normally, that is, to emit signals or to metabolize foreign substances. Thus, it becomes impossible to distinguish between effects due to the substance being tested and those due to autolysis and deterioration of the organ in vitro. See, e.g., Bronaugh and Stewart, J. Pharm. Sci. 74: 64-67 (1985) and Franz, J. Invest. Dermatol.: 190-195 (1975).
The hirsute skin of experimental animals differs fundamentally from the skin of humans in its morphology, its physical properties, and its reactions to allergenic and other stimuli. For example, the rates of percutaneous absorption of animal skin differ considerably from those of human skin. Although, animals will continue to be used to determine LD50 values and the responses to toxic substances of internal organ systems, for many other toxicity studies alternatives to animal testing are being sought, both for ethical reasons as well as for the development of more effective tests (See, e.g., Alternatives to Animal Use in Research, Testing, and Education. Office of Technology Assessment. Washington, D.C. (1985)).
Although cell cultures have many uses as test systems, it has been rigorously shown that the cells grown in monolayer cultures exhibit neither the same biosynthetic repertoire nor the same permeability properties as cells in the organism, nor are they organized or differentiated in the same manner as cells in a tissue or organ.
Thus, alternatives to animal testing and cell culture test systems are being sought. Equivalents of tissue that reproduce in vitro many of the physical and biological characteristics of natural tissues would be useful for the study of the tissue cell biology, physiology and pathology.